[llvm-libtool-darwin] Add support for -V option

The -V option in cctools' libtool prints out the version number and
performs any specified operation. Add this option to LLVM's version.
cctools is more forgiving of invalid command lines when -V is specified,
but I think it's better to give errors instead of silently producing no
output.

Unfortunately, when -V is present, options that would otherwise be
required aren't anymore, so we need to perform some manual argument
validation.

Reviewed By: alexshap

Differential Revision: https://reviews.llvm.org/D86359